[llvm] [AArch64][PAC] Move emission of LR checks in tail calls to AsmPrinter (PR #110705)

[Anatoly Trosinenko via llvm-commits]

================
@@ -1757,38 +1762,70 @@ void AArch64AsmPrinter::emitPtrauthCheckAuthenticatedValue(
   //    Lsuccess:
   //      ...
   //
-  // This sequence is expensive, but we need more information to be able to
-  // do better.
-  //
-  // We can't TBZ the poison bit because EnhancedPAC2 XORs the PAC bits
-  // on failure.
-  // We can't TST the PAC bits because we don't always know how the address
-  // space is setup for the target environment (and the bottom PAC bit is
-  // based on that).
-  // Either way, we also don't always know whether TBI is enabled or not for
-  // the specific target environment.
+  // See the documentation on AuthCheckMethod enumeration constants for
+  // the specific code sequences that can be used to perform the check.
+  using AArch64PAuth::AuthCheckMethod;
 
-  unsigned XPACOpc = getXPACOpcodeForKey(Key);
+  if (Method == AuthCheckMethod::None)
+    return;
+  if (Method == AuthCheckMethod::DummyLoad) {
+    EmitToStreamer(MCInstBuilder(AArch64::LDRWui)
+                       .addReg(getWRegFromXReg(ScratchReg))
+                       .addReg(TestedReg)
+                       .addImm(0));
+    assert(ShouldTrap && !OnFailure && "DummyLoad always traps on error");
+    return;
+  }
 
   MCSymbol *SuccessSym = createTempSymbol("auth_success_");
+  if (Method == AuthCheckMethod::XPAC || Method == AuthCheckMethod::XPACHint) {
+    //  mov Xscratch, Xtested
+    emitMovXReg(ScratchReg, TestedReg);
 
-  //  mov Xscratch, Xtested
-  emitMovXReg(ScratchReg, TestedReg);
-
-  //  xpac(i|d) Xscratch
-  EmitToStreamer(MCInstBuilder(XPACOpc).addReg(ScratchReg).addReg(ScratchReg));
+    if (Method == AuthCheckMethod::XPAC) {
+      //  xpac(i|d) Xscratch
+      unsigned XPACOpc = getXPACOpcodeForKey(Key);
+      EmitToStreamer(
+          MCInstBuilder(XPACOpc).addReg(ScratchReg).addReg(ScratchReg));
+    } else {
+      //  xpaclri
+
+      // Note that this method applies XPAC to TestedReg instead of ScratchReg.
+      assert(TestedReg == AArch64::LR &&
+             "XPACHint mode is only compatible with checking the LR register");
+      assert((Key == AArch64PACKey::IA || Key == AArch64PACKey::IB) &&
+             "XPACHint mode is only compatible with I-keys");
+      EmitToStreamer(MCInstBuilder(AArch64::XPACLRI));
+    }
 
-  //  cmp Xtested, Xscratch
-  EmitToStreamer(MCInstBuilder(AArch64::SUBSXrs)
-                     .addReg(AArch64::XZR)
-                     .addReg(TestedReg)
-                     .addReg(ScratchReg)
-                     .addImm(0));
+    //  cmp Xtested, Xscratch
+    EmitToStreamer(MCInstBuilder(AArch64::SUBSXrs)
+                       .addReg(AArch64::XZR)
+                       .addReg(TestedReg)
+                       .addReg(ScratchReg)
+                       .addImm(0));
 
-  //  b.eq Lsuccess
-  EmitToStreamer(MCInstBuilder(AArch64::Bcc)
-                     .addImm(AArch64CC::EQ)
-                     .addExpr(MCSymbolRefExpr::create(SuccessSym, OutContext)));
+    //  b.eq Lsuccess
+    EmitToStreamer(
+        MCInstBuilder(AArch64::Bcc)
+            .addImm(AArch64CC::EQ)
+            .addExpr(MCSymbolRefExpr::create(SuccessSym, OutContext)));
+  } else if (Method == AuthCheckMethod::HighBitsNoTBI) {
+    //  eor Xscratch, Xtested, Xtested, lsl #1
+    EmitToStreamer(MCInstBuilder(AArch64::EORXrs)
+                       .addReg(ScratchReg)
+                       .addReg(TestedReg)
+                       .addReg(TestedReg)
+                       .addImm(1));
+    //  tbz Xscratch, #62, Lsuccess
+    EmitToStreamer(
+        MCInstBuilder(AArch64::TBZX)
+            .addReg(ScratchReg)
+            .addImm(62)
+            .addExpr(MCSymbolRefExpr::create(SuccessSym, OutContext)));
+  } else {
----------------
atrosinenko wrote:

On the one hand, a dangling 
```cpp
  } else {
    llvm_unreachable("Unsupported check method");
  }
```
block looks a bit clumsy, on the other hand, it looks natural from the standpoint of introducing completely new check methods (if any) by inserting one more `if else (... ) { ... }` block of text, so let's keep as-is for now...

https://github.com/llvm/llvm-project/pull/110705